Definition - Scrambling usually refers to operations carried out in the analog domain. Scrambling is accomplished by the addition of Components to the original signal or the changing of some important components of the original signal in order to make extraction of the original signal difficult.
Purpose - The purposes of Voice Scrambling are to enable accurate timing recovery on receiver equipment without resorting to redundant line coding. It facilitates the work of a timing recovery circuit, an automatic gain control and other adaptive circuits of the receiver (eliminating long sequence consisting of ‘0’ or ‘1’ only). For every dispersal on the carrier, reducing inter-carrier signal interference. It eliminates the dependence of a signal’s power spectrum upon the actual transmitted data, making it more dispersal to meet maximum power spectral density requirement.
Types of Scramblers – There are two types of scrambler are:-
- Additive scramblers (synchronous)
- Multiplicative scramblers (self-synchronizing)
Additive scramblers: Additive scramblers transform the input data stream by applying a pseudo-random binary sequence (PRBS). Sometimes a pre-calculated PRBS stored in the Read-only memory is used, but more often it is generated by a linear feedback shift register (LFSR). A receiver searches for a few sync-words in adjacent frames and hence determines the place when its LFSR must be reloaded with a pre- defined initial state.
Multiplicative Scramblers: Multiplicative Scramblers are called so because they perform a multiplication of the input signal by the scrambler’s transfer function in z-space. They are discrete linear time-invariant systems. A multiplicative scrambler is recursive and multiplicative de-scrambler is non- recursive.
Points to remember
- Both types may fail to generate random sequence under worst case input conditions.
- Multiplicative scramblers lead to error multiplication during de-scrambling.
- Additive scrambling must be reset by the form syc; if this fails massive error propagation will result as a complete frame cannot be descrambled.
Noise - Any Disturbance while transfer of voice signal from one place to another is called Noise.
Voice Inversion Scrambling- Voice Inversion scrambling is an analog method of obscuring the content of a transmission. It sometimes used in public service radio, automobile racing, cordless telephones and the family radio service.
Types of Voice Inversion Scrambling - Various types of voice inversion scrambling are:
- VPU-12A- User programmable voice Inversion scrambler programmable for up to 16 speech inversion codes using $-line binary. It is programmable to any inversion frequency from 2100-4096 Hz.
- VPU-15- Speech inversion voice scrambler with 4 selectable inversion codes. User-programmable features include multiple inversion frequencies, ANI, Deadbeat, Disable, selective call and OTAR.
- VS-1000- Programmable for any inversion code between 2100-4096 Hz .User programmable for upto 16 codes. Programmable via Midian’s KL-4F Programmer.
- Rolling code- Method to use a random code to choose the carrier frequency and then change this code in real time. This is called Rolling code voice inversion and one can often hear the “tricks” in the transmission which signal the changing of the inversion parts.
- Split band voice – Another method is split band voice inversion method sometimes called VSB. This is where the band is split and then each band is inverted separately. A rolling code can also be added to this method of split band variable inversion.
Advantages and Disadvantages of Voice Scrambler
Advantage - It completely takes place within the audio bandwidth of a channel.
Disadvantage - Voice Scrambler has following drawbacks:
- Eavesdropper can easily reverse the process of frequently mirroring with a simple piece of electronics.
- Possible to extract useful information from the seemingly grabbled speech.
- Can be defeated by using today’s software.
Types of Security
- Security is used to hide the data from unwanted persons. Security can be done with the help of following methods:-
- Steganography- Identity based.
- Hiding the parties to a communication - preventing identification, promoting anonymity. One of the best example is ‘crowd’. It is difficult to identify who said what when it comes from crowd.
Tools to obtain Security: Various tools for enabling/maintaining security are:
- Identity Based Networks- Unwanted or malicious behaviour is possible on the web since it is inherently anonymous. True identity based networks replace the ability to remain anonymous and one inherently more trustworthy since the identity of the sender and receiver are known.
- Anonymized Networks- In this technique ethically, a large number of users running the same system, can have communication routed between in such a way that it is very hard to detect what actually the complete message is, which user send and where is its final destination or ultimately going from or to. Example- crowd, TOR, 12P etc.
- Anonymous Communication Devices- As per study, an unknown device would not be noticed, since so many other devices are in use. But in real time scenario things are not the same, due to the presence of system such as Carnivore (FB) and Echelon which can monitor communication over entire network. Eg- payphones, Internet Cafe, etc.
Programs offering more Security: Some of the softwares that provide good security are:- Skype, zfone, I2P-Messenger TrilMan, off-the-Record Messaging.